The present invention relates to a wire sawing device comprising wire guide cylinders supporting a wire maintained in position by grooves provided on the surface of said wire guide cylinders which define the interval between the wires of a layer of wires, the wire being adapted to move with alternating or continuous movement bearing against at least one piece to be sawed fixed on at least one support table.
There are known such devices for cutting thin slices from a piece to be sawed, in which the stretched wire is both guided and tensioned by the wire guide cylinders. These wire guide cylinders are generally covered with a layer of synthetic resin and are scored with grooves whose geometry and size must be very precise. The wire is spirally wound about wire guide cylinders and forms between two wire guide cylinders at least one layer of parallel wires whose distance between two consecutive wires fixes the thickness of the slices. Because of the spiral winding, all the wires of the layer of wires move parallel generating a force perpendicular to the advance of the support table and inducing shearing and torsion forces on the mechanism for holding the piece to be sawed. These forces are a function of the speed of sawing and the dimensions of the piece to be sawed and can be sufficiently great to wrench the piece to be sawed from its support. The tension produced at the base of the piece to be sawed gives rise to deformations which after sawing show up in the form of warping of the obtained slices. Moreover, the plane of the layer of wires is, in the known devices, generally perpendicular to the direction of sawing, which induces undulations on the surface of the slices in the case of overall movement of the layer of wires resulting from thermal oscillations for example. These undulations, even of several micrometers, are enough to render the slices unusable for certain applications such as silica for the semiconductor industry.
The object of the invention consists in overcoming the mentioned drawbacks by permitting the wire sawing device to produce slices of better geometric quality by decreasing the undulations and the warpage of the latter whilst improving the productivity of the assembly. The device according to the invention is characterized to this end, by the fact that it comprises at least four wire guide cylinders arranged in sequence one following the other with two outer wire guide cylinders and at least two inner wire guide cylinders, the wire being arranged on the outer wire guide cylinders at a first interval and on the inner wire guide cylinders at a second interval corresponding to half the first interval, the wire from a first groove on an inner wire guide cylinder passing into a groove of the outer wire guide cylinder to be returned to a second groove on the inner wire guide cylinder spaced from the second interval of the first groove, the wire coacting with the adjacent inner wire guide cylinders by passing from the first upper portion, respectively the lower portion, of one of the inner wire guide cylinders in a direct line to the lower portion, respectively the upper portion, of the adjacent inner wire guide cylinder so as to constitute an X arrangement formed by two layers of wires that cross at a crossing angle at least equal to 20xc2x0 and determined by the diameter of the inner wire guide cylinders and by the spacing separating the axes of the two adjacent inner wire guide cylinders.
There are thus obtained two layers of wires forming a sufficient angle with the cutting direction and whose wires are projected on a working plane defined by the strictly parallel axes of the inner wire guide cylinders, such that the surfaces of the sawed slices will also be parallel. The crossing of the layers at an angle of at least 20xc2x0 will reduce the problem of undulations because the latter will no longer be parallel but will be angularly different on opposite sides of the obtained slices, which can reduce, simplify or even render unnecessary, the subsequent operations of grinding or truing. The use of at least two inner wire guides permits providing in a simple manner, by winding of the crossed wire in an X arrangement, the two sawing directions with a sufficient angle to create the required effect so as to decrease the problem of possible undulations. It is particularly important to note that in this arrangement, the shear forces induced in the support by the two layers having very different angular directions from the sawing direction, hence the advance of the support table, are considerably reduced. Moreover, the angular position of the layers of wires relative to the movement of the support table, and hence of the piece to be sawed, decreases the cutting pressure on the surface of the piece to be sawed and thereby permits a greater cutting speed, increasing at the same time the productivity of the wire sawing device.
Because the layers cross in an X arrangement while passing from the upper and lower portions of the inner wire guide cylinders, the distribution of the forces of the wires on the inner wire guide cylinders is well balanced. The forces on the bearings of the inner wire guide cylinders are thus low. The heat given off by these bearings will thus be decreased, which will permit an improvement of the geometry of the sawed slices. Moreover, there can be used roller bearings of smaller size.
The precision of the sawed slices is also improved because the inner wire guide cylinders are subjected to the smaller forces determining the position of the wires of the crossing layers of wires, whilst the outer wire guide cylinders are subjected to large directional forces that do not influence the position of the wires, but serve for driving.
The necessary angle between the two layers must be at least 20xc2x0 if it is desired to obtain a substantial crossing effect. However, beyond 90xc2x0 the advantage of crossing layers decreases.
Because the layers of crossed wires are stretched in a direct line between the adjacent inner wire guide cylinders to form an X-shaped arrangement, therefore without passing through a guide member such as a guide or deflection cylinder, there is obtained a large crossing angle of the layers and a very good alignment and optimum parallelism of the wires of a layer.
Given that the wires are arranged on the inner wire guide cylinders at an interval corresponding to half that of the outer wire guide cylinders which return the wire received from a groove of the inner wire guide cylinder to a distant groove of a half interval on the inner cylinder, the wires are arranged on the inner wire guide cylinder such that their projections on the working plane are perfectly parallel. Slices with parallel surfaces are thus obtained in a simple manner with the layers crossed at a large crossing angle, greater than or equal to 20xc2x0.
The invention thus permits providing a superior wire sawing device having high productivity, a mean precision increased by the decrease of geometric faults and the decrease of the internal tensions in the course of sawing in the piece to be sawed which is often of a material very sensitive to external forces.
According to a preferred embodiment, the device comprises n inner wire guide cylinders arranged in a sequence following each other, n being a whole number equal to or greater than 2, the wire coacting with the inner wire guide cylinders so as to obtain (nxe2x88x921) crossings of wire layers, (n=1) pieces to be sawed being adapted to be applied against the crossed wire layers substantially adjacent their crossing.
Preferably, n is comprised between 3 and 6.
With these arrangements there is obtained simultaneously a substantial number of advantages. Instead of having a ratio of the number of inner wire guide cylinders to the number of pieces to be sawed equal to 2, this ratio can be considerably reduced, namely to 1.5, 1.33, 1.25 or 1.2, for respectively 3, 4, 5 or 6 inner wire guide cylinders. There is thus obtained a ratio of the wear of the coating of the wire guide cylinders to the number of sawed pieces which is increasingly small and is advantageous. Of course the number of wire guide cylinders cannot be increased indefinitely because of the problems of overall size. Given that the device is capable of sawing several pieces simultaneously, there is obtained a considerable gain in productivity. Simultaneously important advantages are enjoyed because of the crossed layers of wires.
According to a preferred embodiment, the inner wire guide cylinders are arranged in a plane and the support tables are actuated by a common control member.
Because of this arrangement, there is obtained a particularly simple and effective arrangement which requires only a single control member.
According to a modification, each support table is adapted to be moved individually toward the layers of crossed wires or moved away from these, by electrical, pneumatic, hydraulic, magnetic and/or manual actuating means.
This arrangement permits precise sawing of pieces to be sawed of different sizes.
In these modifications, the centers of the axes of the wire guide cylinders can be arranged in a non-rectilinear arrangement.
These characteristics permit reducing the overall size as to the width of the entire device.